1. Field of the Invention
The present invention relates to an intermediate fluid type vaporizer that heats and vaporizes low-temperature liquid, such as liquefied natural gas (hereinafter referred to as "LNG"), by using an intermediate fluid such as propane. The present invention also relates to a natural gas supply method using the vaporizer.
2. Description of the Related Art
As a means for continuously vaporizing low-temperature liquid, such as LNG, in a compact structure, an intermediate fluid type vaporizer is known which uses an intermediate fluid in addition to a heat source fluid (Japanese Unexamined Patent Publication No. 53-5207, etc.).
FIG. 5 shows an LNG vaporizer as an example of the intermediate fluid type vaporizer. The LNG vaporizer comprises an intermediate fluid evaporator E1, an LNG evaporator E2, and a natural gas (hereinafter referred to as "NG") heater E3. The LNG vaporizer further comprises an inlet channel 10, multiple heat source tubes 12, an intermediate channel 14, multiple heat source tubes 16, and an outlet channel 18 that are provided, in that order, so as to form a path through which a heat source fluid (sea water in this example) flows. The heat source tubes 12 are disposed in the NG heater E3, and the heat source tubes 16 are disposed in the intermediate fluid evaporator E1. The intermediate fluid evaporator E1 contains an intermediate fluid (e.g., propane) 17 whose boiling point is lower than that of sea water serving as the heat source fluid.
The LNG evaporator E2 comprises an inlet channel 22 and an outlet channel 24 that are divided by a partition plate 20, and multiple heat transfer tubes 23 that link both the channels 22 and 24. The heat transfer tubes 23 are approximately U-shaped, and project above the intermediate fluid evaporator E1. The outlet channel 24 is connected to the inside of the NG heater E3 via an NG piping 26.
In such a vaporizer, sea water serving as the heat source fluid passes through the inlet channel 10, the heat source tubes 12, the intermediate channel 14, and the heat source tubes 16, and reaches the outlet channel 18. While passing through the heat source tubes 16, the sea water exchanges heat with the intermediate fluid 17 in the intermediate fluid evaporator E1, thereby evaporating the intermediate fluid 17.
On the other hand, LNG to be vaporized is introduced from the inlet channel 22 into the heat transfer tubes 23. The LNG in the heat transfer tubes 23 and the evaporated intermediate fluid 17 in the intermediate fluid evaporator E1 exchanges heat with each other, and the intermediate fluid 17 is thereby condensed. By receiving the heat of condensation, the LNG is evaporated into NG inside the heat transfer tubes 23. The NG is introduced from the outlet channel 24 into the NG heater E3 through the NG piping 26, is further heated by heat exchange with sea water that flows through the heat source tubes 12 in the NG heater E3, and is then supplied to a consumer.
Therefore, this intermediate fluid type vaporizer allows LNG to be continuously vaporized by repeating evaporation and condensation of the intermediate fluid 17.
The above-described intermediate fluid type vaporizer is more compact and has a lower profile than an open-rack type vaporizer. So, for example, even when there are no LNG supply facilities on the land, LNG can be vaporized off shore and supplied to an onshore consumer by installing both this vaporizer and an LNG tank on a ship or a barge plant floating on the sea. When the above-described intermediate fluid type vaporizer is installed on the sea, however, the entire vaporizer shakes together with the ship or the like due to waves or the like, and the level of the intermediate fluid 17 in the intermediate fluid evaporator E1 greatly changes, which may produce adverse effects on vaporization ability. Specifically, the adverse effects are as follows:
A) Exposure of Heat Source Tubes 16
In a case in which the intermediate fluid evaporator E1 shakes to tilt in the transverse direction (in the widthwise direction nearly orthogonal to the axial direction of the heat source tubes 16), as shown in FIG. 6A, or in a case in which the intermediate fluid evaporator E1 shakes to tilt in the longitudinal direction (in the lengthwise direction nearly parallel to the axial direction of the heat source tubes 16), as shown in FIG. 6C, an intermediate fluid level 17a greatly tilts relative to the evaporator E1, and this may incur a risk that some of the heat source tubes 16 will be exposed above the level 17a. In such exposed portions, heat exchange between the intermediate fluid 17 and the heat source fluid is impossible, which significantly impairs vaporization ability.
B) Surface Wetting of Heat Transfer Tubes 23
When the shaking in the longitudinal or transverse direction becomes large, there is a risk that the heat transfer tubes 23 of the LNG evaporator E2 will be wetted due to a wave 17b of the intermediate fluid 17 striking the LNG evaporator E2, as shown in FIG. 6B, or because the end of the LNG evaporator E2 is soaked in the intermediate fluid 17, as shown in FIG. 6C. Such surface wetting significantly impairs the condensation ability of the intermediate fluid 17 on the surfaces of the heat transfer tubes 23.